1. Field of the Invention
The present invention relates to a base band signal processing circuit that processes a base band signal required for digitally modulating a carrier.
2. Description of the Related Art
In the transmission in the CDMA portable telephone, audio signals and various control signals are transformed into a digital signal by means of a spread signal, this digital signal is converted into an analog signal, and using the analog signal the carrier is digitally modulated (modulated in QPSK mode).
Referring to FIG. 3, a conventional base band signal processing circuit will be explained.
First, audio signals, etc., are transformed into a digital signal D by a digital processing circuit not illustrated, which is inputted to a first D/A converter 21 and a second D/A converter 22. The first D/A converter 21 and the second D/A converter 22 convert the digital signal D into analog signals by means of a clock signal. The first D/A converter 21 outputs an analog I signal A/I, and the second D/A converter 22 outputs an analog Q signal A/Q. There is a 90xc2x0 phase difference between the analog I signal A/I and the analog Q signal A/Q.
The analog I signal A/I and the analog Q signal A/Q each have frequency bands of about 630 kHz, and each are inputted to a first low-pass filter 23 and a second low-pass filter 24 that eliminate noises of higher frequencies than that. The first low-pass filter 23 and the second low-pass filter 24 are configured with active low-pass filters that can be integrated into ICs for the downsizing of portable telephones. The analog I signal A/I whose noises over 630 kHz are removed is amplified by a first base band signal amplifier 25, and thereafter inputted to a first modulator 27. In the same manner, the analog Q signal A/Q whose noises over 630 kHz are removed is amplified by a second base band signal amplifier 26, and thereafter inputted to a second modulator 28. The first and second base band signal amplifiers 25, 26 are configured with operational amplifiers.
Further, in order to set the level of the analog I signal A/I inputted to the first modulator 27 and the level of the analog Q signal A/Q inputted to the second modulator 28 to a specific level (for example, 1 volt), the amplification factors of the first base band signal amplifier 25 and the second base band signal amplifier 26 are designed to be varied individually.
In addition to the analog I signal A/I, the first modulator 27 is supplied with a first carrier "PHgr"/I, and in addition to the analog Q signal Q/I, the second modulator 28 is supplied with a second carrier "PHgr"/Q. There is a 90xc2x0 phase difference between the first carrier "PHgr"/I and the second carrier "PHgr"/Q, and they are generated by a phase-shifter 30 using an original carrier (frequency is about 130 MHz) delivered from a carrier oscillator 29.
Thus, the first modulator 27 applies the PSK modulation to the first carrier "PHgr"/I by means of the analog I signal A/I, and the second modulator 28 applies the PSK modulation to the second carrier "PHgr"/Q by means of the analog Q signal Q/I.
The two carriers modulated in PSK mode are added by an adder 31, and then to the two carriers thus added a frequency converter not illustrated executes the frequency conversion into a transmission signal of, for example, 800 MHz band to 900 MHz band. Thereafter, specific processing is further applied to the transmission signal, which is transmitted to the base station from an antenna not illustrated.
In the base band signal processing circuit explained above, aiming at the downsizing of the portable telephones, various circuits are integrated into ICs, and the scale of integration has been expanded. In addition, it has been pursued to lower the power supply voltage. This lowering of the supply voltage will inevitably deteriorate the noise characteristic S/N and the distortion characteristic S/D in the first and second D/A converters 21, 22, the first and second low-pass filters 23, 24, the first and second base band signal amplifiers 25, 26, and the like.
As a measure for this problem, to swell the level of the base band signal will improve the noise characteristic S/N, but it will deteriorate the distortion characteristic S/D. On the contrary, to reduce the level of the base band signal will improve the distortion characteristic S/D, but it will deteriorate the noise characteristic S/N. Thus, the noise characteristic S/N and the distortion characteristic S/D are on the counterbalance; and therefore, it has been a great problem how to arrange a compromise of improvement between these two characteristics.
Accordingly, it is an object of the present invention to provide a base band signal processing circuit that improves both the noise characteristic and the distortion characteristic as well as sets each of the characteristics in a well-balanced state.
In order to accomplish the foregoing object, the base band signal processing circuit of the present invention contains a first D/A converter that converts a digital signal into analog signals and outputs them as first base band signals, a second D/A converter that converts the digital signal into analog signals and outputs them as second base band signals having a phase shifted by 90xc2x0 against the first base band signals, a first active low-pass filter, and a second active low-pass filter. In this construction, the first active low-pass filter and the second active low-pass filter are each configured in a balanced type, the first base band signals are differentially outputted from the first D/A converter and differentially inputted to the first active low-pass filter, the second base band signals are differentially outputted from the second D/A converter and differentially inputted to the second active low-pass filter, the first base band signals are differentially outputted from the first active low-pass filter, and the second base band signals are differentially outputted from the second active low-pass filter.
Further, in the base band signal processing circuit of the present invention, a first balanced type variable attenuator is provided between the first D/A converter and the first active low-pass filter, a second balanced type variable attenuator is provided between the second D/A converter and the second active low-pass filter, the levels of the first base band signals inputted to the first active low-pass filter are made to be varied by the first balanced type variable attenuator, and the levels of the second base band signals inputted to the second active low-pass filter are made to be varied by the second balanced type variable attenuator.
Further, the base band signal processing of the present invention contains a first balanced type variable gain amplifier and a second balanced type variable gain amplifier. In this construction, the first balanced type variable gain amplifier amplifies the first base band signals outputted from the first active low-pass filter to output in the differential mode, the second balanced type variable gain amplifier amplifies the second base band signals outputted from the second active low-pass filter to output in the differential mode, and the amplification factor of the first balanced type amplifier and the amplification factor of the second balanced type amplifier are to be varied.